Digital printing, including inkjet and electrostatic printing, is often used to produce a series of identical images on one or more substrates. Different colors of marking materials (e.g., ink or toner) typically have different pile heights that extend above the substrate. In addition, many images will have areas that include marking material and other areas that include no marking material. Therefore, pile height differentials are typically encountered across an image printed on the substrate. When an image is printed repeatedly, the pile height differentials add up as the printed images accumulate in an output area. The accumulated pile height differentials can lead to distortions in the output media (e.g., a roll or stack of media) and these distortions may cause disruptions in subsequent workflow operations.
One example of a situation where pile height differentials may cause disruptions is with roll-to-roll printing applications. The roll-to-roll format is commonly used for printing on flexible packaging substrates, such as films and foils, which are subsequently used downstream for food packaging and other packaging applications. With roll-to-roll printing, a length of media in the form of a print substrate is fed from an input roll to a printing device. The printing device prints images on the substrate and the substrate is then fed to an output roll. When the thickness of the layer of marking material printed on the substrate is substantial (e.g., the thickness of the ink layer approaches the thickness of the substrate), it can introduce distortion to the output roll which may disrupt normal operations. In particular, if the cumulative pile height of the marking material is not relatively consistent across the roll, one side or a portion of the output roll may become unbalanced. For example, if an image printed on the right side of a substrate contains substantial print content, while the image printed on the left side of the substrate contains only limited print content, the right side of the substrate will have a greater cumulative pile height over time, and the right side of the output roll will end up with a greater diameter than the left side of the output roll. In addition, the right side of the roll will tend to be taut while the left side of the roll will tend to be loose. When the same or similar image is repeatedly printed, as is typically the case with roll-to-roll printing, this repetition only magnifies the pile height problem at the output roll. Distortion in the output roll creates problems during both the printing process and downstream in the packaging process.
Another example of a situation where pile height differentials may cause disruptions is with sheet stacking applications. In sheet stacking applications, the same image may be printed repeatedly on sheet after sheet. If a regular and relatively large pile height differential is found on a specific part of each page, the stack of sheets output from the printing device may be distorted as the pages accumulate in the output stack. For example, if the pile height on the right side of each page is relatively high, while the pile height on the left side of each page is relatively low, the stack of pages will become unbalanced, with the right side of the output stack higher than the left side. This distorted output stack situation may be even more pronounced when the print substrate is relatively thin in a sheet stacking application, as is often the case with books or catalogues. Distortion in the output stack may eventually create problems with subsequent workflow, such as when the stack of pages needs to be handled or otherwise manipulated after printing. Binding the stack of pages into a book or catalogue can be particularly difficult if the height of the stack is higher on one side of the sheets than on the other, or if the height of the stack is generally uneven across the sheets.
In view of the foregoing, it would be advantageous to provide a method of printing images to compensate for pile height differentials.